1. Field of the Invention
The present invention relates to a semiconductor memory device and a method of fabricating the same.
2. Description of the Related Art
A non-volatile random access memory (RAM) that maintains last data even though a power supply is turned off and allows random access is regarded as a promising device that forms the foundation of the ubiquitous society in the fields of personal identification and security.
Among various non-volatile RAMs, a non-volatile memory (FeRAM) that uses a ferroelectric having spontaneous polarization in the dielectric layer of a capacitor is expected as the next generation memory in the field of mobile devices because its power consumption is small.
Particularly, a one transistor (1T) FeRAM having a structure in which a ferroelectric film is provided on a gate insulating film of a field effect transistor (FET) can be subjected to nondestructive readout, and is characterized in that the cell area can be made smaller. On this account, the 1T FeRAM is advantageous in respects of the number of readouts and higher integration over a one transistor and one capacitor (1T1C) FeRAM and a 2 transistor and 2 capacitor (2T2C) FeRAM subjected to destructive readout.
A typical 1T FeRAM has the FET structure in which a gate insulating film (I), a ferroelectric film (F) and an electrode film (M) are layered on the channel region of a semiconductor substrate (S) having a source region and a drain region formed thereon. The FeRAM having this structure is called an MFIS (a metal layer (Metal), a ferroelectric layer (Ferroelectric), an insulating layer (Insulator), and a semiconductor layer (Semiconductor) FeRAM. In the MFIS FeRAM, the ON-state current value is different from the OFF-state current value in a few digits between the source and the drain depending on the orientation of polarization of the ferroelectric film, and this is used to determine whether to be the “0” state or the “1” state.
For the ferroelectric film, the following is used: Pb(Zr, Ti)O3 having a perovskite structure, or PZT materials in which Pb(Zr, Ti)O3 is added with a dopant such as La, Ca, Sr, or Nb, or (Bi, La)4Ti3O12 or SrBi2Ta2O9 having a bismuth layer structure. The ferroelectrics having the bismuth layer structure have a polarization charge amount as well as a dielectric constant smaller than those of PZT materials. The FET FeRAM operates adequately as long as the polarization charge amount of the ferroelectric film is about 1 μC/cm2. Because the ferroelectrics having the bismuth layer structure have a small dielectric constant to which a large voltage can be applied, the ferroelectrics are often used for the ferroelectric layer.
The 1T1C FeRAM is already commercially practical. However, for the 1T1C FeRAM, a single transistor and a single ferroelectric capacitor are required for a single memory cell, and thus a certain size is necessary for the 1T1C FeRAM to form memory cells. The charge amount that can be stored in the ferroelectric capacitor is proportional to the area, and the charge amount necessary for storage cannot be maintained as scale-down is being advanced. In other words, in the 1T1C FeRAM, it is difficult to fabricate a large capacity memory.
On the other hand, the FET FeRAM is capable of conducting nondestructive readout, and advantageous in that the cell area can be made smaller as compared with the 1T1C FeRAM. However, in the current FET FeRAM, write and read voltages are high, and the reliability of the insulating film is poor. Therefore, the FET FeRAM has the retention properties and the imprinting properties indicating the storage performance behind those of the current 1T1C FeRAM. On this account, the FET FeRAM is not commercially available yet.
The Patent Reference 1 (JP-A-2004-172483), in order to exhibit both of ferroelectricity and ferromagnetism, such a semiconductor memory device is disclosed that the semiconductor memory device has a ferroelectric film and a ferromagnetic film formed of separate materials and layered. In this semiconductor memory device, it is necessary to layer the ferroelectric film and the ferromagnetic film in such a way that the electric field occurring in the ferroelectric film and the magnetic field occurring in the ferromagnetic film are affected to each other. In order not to provide the multilayer structure of the ferroelectric film and the ferromagnetic film, such a method is also considered that a ferromagnetic material is mixed with a ferroelectric material for pressure forming. However, in such a method, the properties and reliability of the ferroelectric film and the ferromagnetic film might be impaired.